View Full Version : Report: Science Fair "Digital Computer Kit"

June 8th, 2008, 05:11 AM
"And then what'd he do?" An informal student of psychology grapples with technology.

The PBS shows on brain health that I watched all said that one of the ways to retain your mental capacity is to engage in activities that provide challenges that are difficult for you to solve. Around the same time, my interest in vintage computers was resurgent, so I thought that I would make another attempt to strech myself by pursuing that interest, by taking it to the next level, so to speak. This post is a train-of-thought description of what happened when I did that, meant to be humorous. I don't mind if people just skip to the next message. It is provided for any persons who think than an illumination of the way I think sounds interesting.

Still reading? Good, let's commence. Last time I was pursuing vintage computers I fancied myself as a budding computer programmer. I bought all of the accoutraments off of eBay, including an original IBM PC for $15. I started with a Radio Shack Science Fair Microcomputer Trainer to get a feel for assembly language. I would sit listening to Wendy Carlos' soundtrack to the movie "Tron" and write code. But, mysteriously, when I got to end of the first of three sections of the manual I just stopped. I guess that wasn't really what I resonated with. One of my programs achieved the objective in fewer bytes then the example in the book, which is what all programmers want to do, but I guess that wasn't enough.

Now it's three years later and I am concentrating on the hardware more than creating software. This time I bought a Science Fair "Digital Computer Kit". This thing is a bunch of switches and wires that together perform logical operations that I suspect approximate what goes on inside your computer's Central Processing Unit.

So today was a rare day. There was nothing on eBay to bid on today. I took that as a sign that I should try actually using some of the stuff I bought on eBay. I don't know what the definition of shopping addiction is, but I suspect that I have it - I'd much rather be winning bids at low prices at the last minute if I had my choice.

I took a look at the "Digital Computer". I noticed that it was missing the template for the first experiment. "That's O.K." I decided - I could look at the drawing of the template on the wiring diagram to see how to interact with the computer after I wired it up. Did I mention that there were about a million wires that had to be hooked up to perform the experiment? No, well, I am getting ahead of myself.

First I had to test the computer to see if it had been assembled properly. O.K., bulb test. Great, all but two of them work. Where am I going to get replacement bulbs? Ah, worry about that later - for noe, proceed with the switch test. Hmmm, fewer bulbs lit up this time. I wonder if there are any loose connections? I turn the computer over and notice that the wire to one of the bulbs hasn't been crimped into the common harness. O.K., there is a little bit of wire coming out of the end of the bundle.

I try to twist the errant wire to the one sticking out. It takes three attempts before it stays attached. I perform another bulb test to see if I've solved the problem. Now about half of the bulbs don't light up. I look at the wires leading to them. They are soldered, no doubt at the factory. I take the bezel and window off of the computer and try wiggling one of the bulbs. It wiggles pretty good - it has come unscrewed. I tighten all of the bulbs that are loose and perform another bulb test. They all light up: awesome, look at me!

I go on to the switch test. Half of the bulbs fail to light when there switches are closed. I go back to testing those bulbs. The same ones fail to light. I hadn't replaced the window, so I try tightening the bulbs in their sockets. How in the world did they get loose from just turning the computer over?

I get them tightened and they all light until I do the switch test again. What the? I tighteb the bulbs again and the same thing happens _again_. This time I try to calculate how tightly I can grasp the bulbs without breaking them so I can get them really tight. They all light up on the switch test. Yes!

At this point it occurs to me that various bits of Radio Shack equipment bought off of eBay have shown a trend over the years. The oldest piece I ever evaluated was from the early seventies, judging by the copywrite date on the manual. The general quality of Radio Shack equipment was poor back then, getting a little better in the late seventies, and finally getting to be good quality in the eighties. I guess that I was lucky to have grown up and to have become interested in electronics when the quality was good. The computer's copywrite date is 1977.

I return from this reverie to discover that I haven't tested the switches properly. I was supposed to push the switch and touch a hot wire to every other spring contact in each of the ten rows of switches. Then pull the switch back and test the other contacts. I do this, noting that one of the contacts fails to light it's bulb.

Back to the manual. Oh, the switches weren't supposed to be installed until after this test was performed. At this point I feel really sorry for the poor kid who probably got this computer for a present. I guess the poor little guy was thinking like a man - we don't need no steenking directions! I really feel sorry for him. He probably didn't have any mechanism to ease the pain of failure and probably felt really bad that he couldn't get the thing to work.

At this point I consider that these "educational" kits from Radio Shack probably have a much sinister purpose. That is, rather than teaching young people new skills, they instead separate the capable from the not-so-capable. In other words, if you have what it takes to be an electrical engineer, you will use those skills to succesfully complete the kit and it's experiments, be encouraged by this, go to college, get a degree, and go work for intel designing C.P.U.s. But if you don't have what it takes you become a liberal arts major and, hopefully, when it comes time to sink or swim in the job market, you find your way. But then I think that the engineers who designed this kit really did want to pass along their art to the next generation and they did their best to realize that goal in the creation of this kit. I wonder why these things are no longer on the market...

Flip the computer over and pry one of the four latches to let the back of the switch free. The manual says I should check the placement of a small and large spring inside any switches that fail to operate. It breaks - the plastic has "dried out" in the thirty or so years since it was molded. Oh well, I had originally wanted to just make a list of which contacts don't work and re-wire them during the experiment phase. So I write the list. And promptly loose it. But I don't realize that until later.

Now I am ready to attach the wires to the various switch contacts to define the logic of the first experiment. But it is time to watch the eleven o'clock news, so I take a break. Saturday Night Live comes on. I try not to get sucked into this repeat of a show from last November, unsuccessfully. I listen to the opening of the show, then start wiring the computer's logic circuits.

I keep telling myself that I have to develop a systematic approach to attaching the wires. Just about the time that I develop it, I have worked to the rows that had all their connections attached while wiring up the earlier rows on the left side of the board. I decide to be a good worker and verify all of the contacts. All good. But now it's time to eat.

I flip a few of the switches to see if I get a warning light, which I thought would indicate that the circuit is wired up correctly. Only one of the switches has any effect. Hmmm...well, I will eat and then try to figure it out.

I eat my vegetables (another way to prevent Alzheimer's disease). They don't taste bad like I thought they would when I decided to force myself to heat them up and eat them. They never do, but every time that I am suppossed to make them I think they will. I wish I could remember that.

Fortified, I return to the kit. I have to stand up to work on it in the light. My treadmill blocks the light from reaching the area where my chair is. I trace the path that the electricity is suppossed to follow to light the bulb. I wonder for the seventh time that night if I have wired the batteries into a short-circuit. I feel the area below and on top of the battery compartment, but neither is hot. I wish that I had had the foresight to purchase a couple of old-fashioned carbon-zinc batteries that don't have as much energy as the alkaline batteries that I am using. Somebody must still make them - I got a set with a reel to reel tape recorder that I had bought off of eBay. But, apparantly, I haven't messed up yet.

By tracing the circuit that the wires make and tracing it on the wiring diagram I see that I have failed to place at least two wires. I fix my mistake. What about my "systematic approach" that I had thought up? I have no idea why it didn't work.

I finish wiring and tentatively test the circuit. There is a master switch that when depressed send current through the logic circuit. I press it and I get four bulbs. Seems like a lot - I had expected one bulb after I pushed one switch. I look at the diagram page to read what the experiment is. It is a simulation of a river crossing.

A farmer has a wolf, a goat, some cabbage, and a rowboat. He needs to get them to the other side of the river intact, but can only carry himself and one other thing in the boat at a time. I consider the problem, remembering the astro-biologist Carl Sagan's claim that ancient people were just as smart as moderns. It's obvious that the goat needs to be removed from the company of the other two things (the wolf who would eat the goat and the cabbage that the goat would eat). I push the switches to indicate that the farmer and the goat have crossed to the right bank of the river. So far, so good. I pull the farmer's switch to indicate that he has returned to the left bank, then push his switch and the cabbage switch to indicate that he has taken that item across. The "Danger on the right bank" light illuminates. Hmmm, looks like you can't leave the goat alone with either item.

I consider the relative worth of each item, and it seems me that the goat couldn't eat very much cabbage while I went back to fetch the wolf. But the farmer lived a different economic reality - he had to get all of his goods to market just to survive to put a new crop in next season. I can't figure out how to do it.

I draw a diagram of the situation on the kitchen counter, with a continuous line indicating the travels of the farmer. It occurs to me that he could bring things back to the left bank and that must be part of the solution. I diagram it out and I think that I have the answer.

I return to the well lit area of the frontroom and start pushing switches. But it is too hard to remember which position represents which item. I take the stack of the bulb masks for the other ninty nine experiments and make marks on the piece of paper to indicate where I should write the names of the items. I do so, consulting the manual. I cut that part of the paper off and tape it to the computer.

I start pushing switches. I have already forgotten how I did it on paper. It occurs to me that the main thing that I am learning is the "expense", in time, of designing a C.P.U.M and how it must be able to be very useful to justify the expense. I get everything across the "river".

I've done it! Success! About two seconds later I have already forgotten, again, how I did it. But at least I know I did it. I suddenly feel, for the first time, that I would like to do the rest of the experiments in the manual. Before I had been thinking that I would try to analyse how the first one worked and then would move on to other things. But the bug has bit - I am an amatuer computer engineer.

I wonder though, since that the farmer has gotten everything across the river, now what does he do?

Sean Kelly

June 8th, 2008, 10:28 AM
I wonder though, since that the farmer has gotten everything across the river,
now what does he do?

He sells the wolf in the nearest market, and with the money, he buys himself a
nice vintage computer on epay!! ...Cabbage and goat for peripheral$...


June 8th, 2008, 11:15 AM
You need a girlfriend. Maybe two :)

June 8th, 2008, 11:39 AM
ROFL! Do we have a "longest post in history" award? huh, huh?

June 8th, 2008, 12:57 PM
You need a girlfriend. Maybe two :)

O.K. I choose Senator Maria Cantwell and the woman who used to be my doctor to be my core duo. :)


June 8th, 2008, 08:51 PM
O.K. I choose Senator Maria Cantwell and the woman who used to be my doctor to be my core duo. :)


So? What are you waiting for?

June 8th, 2008, 10:50 PM
A farmer has a wolf, a goat, some cabbage, and a rowboat.
I liked Flops. story and the farmer puzzle reminded me of an old article - so I went to looking it up. In my Nov. 1955 issue of Popular Electronics is the same puzzle, except there it's a hen, a wildcat, and some corn. The article discusses the puzzle and also how to make yourself a dandy 'Puzzle-Tronics' circuit out of some junkbox parts to play these logic games. I put the article up here for anyone who wants to see, read, or build it;

June 9th, 2008, 07:13 AM
Hey, as long as we are putting up articles towards building things (and I have looked), how about an AT (not ATX) power supply voltages testing circuit (similar to those little boxes that they have for the ATX PSU) and a device to test 16K x 1 DRAM?

Please and thank you

August 6th, 2008, 12:55 PM
I have a Science Fair 'Digital Computer Kit" that I purchased off of ebay a year ago.

The best method I found for getting all of the wires in place is to photocopy each project page. I then highlight each wire on the photocopy after I've connected it. This way I can see which wires still need to be added.

I've done the first 25 projects in the book - they take some time to do. It seemed like the first few projects in the book were amoung the most difficult to wire. I'm very tempted to replace the bulbs with white LED's, but want to keep the computer in origional condition....

I started to write a "Digital Computer Kit" emulator in Java and kinda-sorta have it working. The big problem with it is that it does not have a user interface - it is just text. I load the "wiring" into a hard coded array, hard code which switches are set to an on position, and then run the problem. It outputs text that indicates which lamps are on.

Now I'm learning how to program ActionScript on my Macintosh - so thinking about redoing this with a user interface and ActionScript....

Terry Yager
August 7th, 2008, 03:47 PM
Whhaaaa...??? Ya mean I'm not the only one who read this? Damn, what a buncha geeks!

BTW, do wolves eat cabbage? What about farmers? How can we trust the farmer not to eat the cabbage? What with all that rowing, seems like he'd be pretty hungry...